The invention relates generally to precision control of an exposure and more particularly to modeling the light output of a light emitting diode (LED) to maintain a constant exposure as the light output of an array of LED""s changes over temperature and the lifetime of the part.
High quality image capture such, as grayscale and color imaging, needs a precision light source. Because of their size, price, reliability, and other qualities, light emitting diodes (LED""s) may be chosen as the light source for image capture. Unfortunately, the light output of an LED changes with junction temperature and age. Because LED""s heat up when they are on, one of the factors that determines the junction temperature of an LED, and hence its light output, is the amount of time, and duty cycle, that the LED is on. One way to compensate for at least part of this variation is to use a light calibration strip. A light calibration strip may be used with a search algorithm to set the illumination levels prior to image capture. A disadvantage of this method is that part of the image capture array is used to sense the calibration strip. This decreases the width or area that is captured at any given moment. Another disadvantage is that this method does not account for changes in the junction temperature during image capture.
Accordingly, there is a need in the art for an illumination compensation. method and apparatus that does not utilize a light calibration strip.
An embodiment of the calculates the LED light output given a set of calibration values, the ambient temperature, the recent on-off history of the device, and the long term history of the device. Once the LED light output is calculated, an exposure can be calculated that will be nearly constant over the operating conditions and lifetime of the device. The ambient temperature is obtained from a temperature sensor. The recent on-off history of the device is timed with a counter-timer. The long-term history of the device and the calibration values are recorded in non-volatile memory. The calibration values are measured and set at the time of manufacture and ensure that each device produces nearly the same exposure under the same conditions.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.